1. Field of the Invention
The present invention relates to a pyroelectric detector using a pyroelectric material element in which electric charge is generated according to the change in the amount of incident infrared light so that movement, temperature and other conditions of an object such as the human body for example can be detected.
2. Description of the Prior Art
As is well known, the responsivity (Rv) of a pyroelectric detector is represented by the following equation (1). ##EQU1## .eta.: emissivity .lambda.: pyroelectric coefficient (dPs/dPT)
.omega.: chopping frequency PA1 A: light receiving area PA1 R: synthesized resistance PA1 G: thermal diffusion constant PA1 .tau..sub.E : electric time constant PA1 .tau..sub.T : thermal time constant PA1 C: capacitance PA1 t: thickness of a pyroelectric substrate PA1 C.sub.p : specific heat
As indicated in the U.S. Pat. No. 3,839,640 and the British Patent Specification No. 1,447,372, a so-called dual element type with series opposite polarity connection in which two light receiving electrode portions are electrically connected in series and in opposite polarities on a pyroelectric material substrate is widely utilized for detection of movement of the human body, and a type having low frequency and high responsivity is preferred.
FIGS. 1 and 2 are schematic views of pyroelectric detectors of the conventional dual element type with series opposite polarity connection.
In FIGS. 1 and 2, the reference character 1 denotes a pyroelectric material substrate formed of a material such as a Pb(Zr, Ti)O.sub.3 ceramics, PbTiO.sub.3 ceramics or SrBaNb, electrodes 2a and 2b on the light receiving side being formed on one surface thereof and electrodes 3a and 3b being formed on the other surface thereof to be opposed to the electrodes 2a and 2b. In a structure shown in FIG. 1, infrared ray absorbants 5 and 6 are formed on the electrodes 2a and 2b, while in a structure shown in FIG. 2, infrared ray absorbants 5 and 6 are also formed on the electrodes 2a and 2b. Further, in the structure shown in FIG. 1, the electrodes 2a and 2b are connected by a conductor 4, while in the structure shown in FIG. 2, the electrodes 3a and 3b are also connected by a conductor 4. The polarization axis of the pyroelectric material substrate 1 are shown by the arrows. The reference character 7 denotes an FET, a gate 8 of which is connected to the electrode 3a in the structure in FIG. 1, the electrode 3b being connected to a grounding terminal 9. In the structure shown in FIG. 2, the gate 8 of the FET7 is connected to the electrode 2a and the electrode 2b is connected to the grounding terminal 9. The reference character 10 denotes a drain and the reference character 11 denotes a source.
DC voltage is applied from the drain 10 and radiant energy enters either the electrode 2a or the electrode 2b, or both of them successively, causing change of temperature in the pyroelectric substrate 1 with respect to the ambient temperature. Then, electric charge is instantaneously generated in the pyroelectric material substrate 1 due to the pyroelectric effect, and electric current flows in the resistance synthesized by a resistor Rg connected between the electrodes 3a and 3b, a resistor of the pyroelectric material substrate 1 and an input resistor of FET 7, so that voltage according to this synthesized resistance is generated. This voltage is subjected to impedance transformation by a source follower circuit of the FET 7 and is superposed on DC bias voltage in the form of voltage change in both ends of the resistor Rs so that an AC output signal is provided from the source 11.
In the example in FIG. 1, the electrode 3a has the minus (-) polarity with respect to the gate 8; the electrode 2a opposing to the electrode 3a has the plus (+) polarity; the electrode 2b connected to the electrode 2a by the conductor 4 has the minus (-) polarity; and the electrode 3b opposing to the electrode 2b has the plus (+) polarity.
In the example in FIG. 2, the electrode 2a has the minus (-) polarity with respect to the gate 8; the electrode 3a opposing to the electrode 2a has the plus (+) polarity; the electrode 3b connected to the electrode 3a by the conductor 4 has the minus (-) polarity; and the electrode 2b opposing to the electrode 3b has the plus (+) polarity.
Concerning the above described equation (1), the following equation (2) is established in case of .omega..tau..sub.E &gt;&gt;1 and .omega..tau..sub.T &gt;&gt;1 because of G=h.omega..sub.T, 1/CR=.omega..sub.E and H=AC.sub.p .multidot.t. ##EQU2## .epsilon..sub.o : vacuum permittivity .epsilon..sub.r : specific inductive capacitance
Accordingly, a small specific inductive capacitance (.epsilon..sub.r) is preferred for a material of the pyroelectric material substrate 1. However, the Pb(Ti, Zr)O.sub.3 ceramics, PbTiO.sub.3 ceramics or SrBaNb shown as a material of the pyroelectric material substrate 1 has a specific inductive capacitance (.epsilon..sub.r) as large as 200 or more and therefore, it is difficult to improve the responsivity (Rv) from a material stand point.
In addition, as seen from the right side of the equation (2), C is preferably as small as possible and since the pyroelectric detector is of the temperature changing type, C.sub.p and t must be small. However, change of .epsilon..sub.r, A and t is limited by the characteristics and accordingly, it must be considered to design a detector having a small capacitance C.
Since the detectors shown in FIG. 1 or FIG. 2 are of a dual element type with series opposite polarity connection, the capacitance C between the gate 8 and the grounding terminal 9 is 1/2 as compared with the capacitance of a single-device type in case where a pair of opposed electrodes are formed on both surfaces of a pyroelectric material substrate. However, the detectors as shown in FIG. 1 or FIG. 2 have problems in that sufficient output cannot be obtained as voltage because the capacitance C is still large and thus charging by pyroelectric current continues too long.
Furthermore, irregularities are sometimes caused in the capacitance of the two devices in a process of formation of electrodes or in the thickness of the infrared ray absorbant in a manufacturing process thereof, resulting in a difference of responsivity between the two devices. As a result, the characteristics of a pyroelectric detector with the series opposite polarity connection type can not be sufficiently utilized as is understood from the facts that:
(1) light coming from outside such as sunlight cannot be sufficiently canceled by two devices,
(2) vibrating noises cannot be sufficiently cancelled by two devices, and
(3) electric charge generated due to the change in the ambient temperature cannot be sufficiently canceled by two devices.